High impact modified polystyrene (sometimes abbreviated as HIPS) is a commercially important commodity used for forming articles in, for example, extrusion molding, compression molding, injection molding, and thermoforming operations to produce articles for which toughness and strength are required. Super high impact polystyrene (SHIPS) materials have been introduced. A SHIPS material will have an impact strength of at least about 3 foot-pounds per square inch as measured by notched Izod impact or about 250 inch-pounds as measured by Gardner impact.
While the impact strength is important in such articles, it is often desirable to be able to form an article that also has high surface gloss. Heretofore, it has been difficult to economically produce such impact-modified, high gloss polystyrene compositions because increased impact strength in rubber modified polystyrene materials has been accompanied by decreased surface gloss. For instance, it is known that increased rubber particle size increases the Notched Izod impact strength of the HIPS. Increased rubber particle size, however, has been observed to decrease the gloss surface in articles formed from HIPS.
One solution to the problem of how to obtain high impact strength, high gloss compositions has been to blend together two HIPS resin, for example one with particle diameters below one micron and one with particle diameters above about 1 or 2 microns. Such compositions are described, for example, in U.S. Pat. Nos. 4,146,589; 4,334,039; 4,421,883; 4,493,922; 5,039,714; and 5,295,656, each of which is incorporated herein by reference, as well as in other publications, such as Styrene Polymers, in 16 Encyclopedia of Polymer Science and Technology 42-45 (H. F. Mark et al. eds., 2d ed. 1987) and Bragaw, The Theory of Rubber Toughening of Brittle Polymers, in Multi-Component Polymer Systems (American Chemical Society Advances in Chemistry Series No. 99, 1971), the disclosures and internal references of which are incorporated herein by reference. However, it would be desirable to increase the impact strength of these blends still further.
It is also known that general purpose polystyrene (that is, polystyrene that is not impact modified) can be blended with styrene block copolymers to improve clarity and haze characteristics and to increase the impact values of the general purpose polystyrene, as is disclosed in U.S. Pat. Nos. 3,906,058; 3,907,931; 4,195,136; 4,239,859; and 4,593,063, each of which is incorporated herein by reference. These blends do not, however, have the same high impact strength of HIPS and SHIPS polystyrene compositions. Whereas the HIPS and SHIPS materials have discrete rubber particles that can affect craze initiation and craze or crack propagation in an impact, the blends of general purpose polystyrene and styrene block copolymer rubbers have some compatibility with one another that prevent formation of small, discrete particulate rubber domains. Thus, typical HIPS and SHIPS materials can have impact values that are about an order of magnitude higher than typical polystyrene-rubber blends.
We have now discovered that polystyrene materials with high notched Izod impact, high Gardner dart impact, and high part surface gloss at 60.degree. may be made by blending together (A) a polybutadiene and/or polybutadiene-based rubber-modified polystyrene polymer with (B) at least one thermoplastic styrenic block copolymer with a styrene content of at least about 70% by weight, based upon the weight of the block copolymer.